The present invention relates to improved photovoltaic cells utilizing titanium dioxide powders consisting of porous particles, ranging in size from 0.1 to 10 microns (10.sup.-6 meter), which possess relatively high bulk density combined with high surface area.
Photovoltaic cells are devices which convert radiant photon energy directly into electrical energy and are commonly used today in small electronic devices such as calculators and watches. These cells are manufactured in a variety of configurations, but generally comprise a layered structure on a substrate. Conventionally, a transparent electrically conductive material (known as an "electrode") is first deposited on a substrate, onto which is deposited a semiconductor material, followed by one or more layers of semiconductor and/or insulator material and/or conductive material. The last functional layer opposite the substrate must be a second electrode, i.e., transparent electrically conductive material. In use, radiant photon energy shines on the surface of the photovoltaic cell causing electrons to move between the electrodes on the cell. The movement of electrons creates an electrical potential difference and, therefore, the generation of an electric current.
Titanium dioxide films are notable for their semiconductive properties and, as such, are useful as the semiconductive components of photovoltaic cells. However, conventional titanium dioxide has little absorbance of light in the visible region and often needs to be combined with or coated with a photosensitive material, such as a dye or chromophore, which absorbs light in the wavelengths emitted by the sun.
EP 407 182 discloses a multilayered photovoltaic cell in which nanometersized titanium dioxide, prepared from colloidal solution, is utilized as a semiconductive layer. While some level of conversion is reported for the titanium dioxide alone, it is disclosed that niobium doped titanium dioxide gave superior results and it is suggested that dye sensitized titanium dioxide and/or doped titanium dioxide is the optimum choice. Given the recognized absorbance deficiencies of titanium dioxide, much work has been devoted to the study of photovoltaic cell configurations and dyestuff additives to improve the absorbency of titanium dioxide in the visible range. For example, WO 91/16719 discloses doping the titanium dioxide with a divalent or trivalent metal to enhance the absorbency. U.S. Pat. No. 5,350,644 discloses a further variation on utilizing doped titanium dioxide wherein a multiplicity of layers of titanium dioxide are formed on the substrate of the photovoltaic cell with the requirement that a dopant be applied to the outermost titanium dioxide layer and further that a photosensitizer be additionally applied to the dopant-containing titanium dioxide layer. U.S. Pat. No. 5,441,827 describes photovoltaic cells using titanium dioxide as the semiconductive material, wherein the colloidal particles making up the titanium dioxide layer have a diameter which is smaller than the diffusion length of the minority charge carriers. Such a restriction requires that the diameter be less than about 100 or 200 nanometers (10.sup.-9 meter) and particles in the size range of 15 nanometers were found to be the optimal size for photovoltaic application. See, e.g., Gratzel et al., "A low-cost, high efficiency solar cell based on dye-sensitized colloidal TiO.sub.2 films", NATURE, Volume 353, pages 737-738 (Oct. 24, 1991).
The colloidally prepared titanium dioxide utilized in the art to date has some well-recognized disadvantages due to the very small particle size. For example, this nanoparticle titanium dioxide is very difficult to handle due to the low bulk density and tends to become easily airborne requiring special safety precautions in handling. When stored as colloidal solution, the shelf life is short. This is due to the aggregation of TiO.sub.2 particles which precipitates out of the solution.
This invention provides for an improved photovoltaic cell utilizing a highly porous, high bulk density titanium dioxide in the size range larger than those disclosed in the art. The use of large, porous TiO.sub.2 particles allows for safe and easy handling of the particles and also retains the high photovoltaic efficiency of nanometer-sized TiO.sub.2, a combination of the best of both materials.
Known solution based methods to produce titanium oxide powders tend to be inefficient with respect to pounds of product per reactor volume, washing of the precipitated products to reduce anions to an acceptable level, and filtration time. The present invention improves upon all of these deficiencies of the prior art to provide processes with greater volume efficiency to achieve more product per reactor volume. The present invention also provides processes that require less water and less time to wash the products to acceptable ionic conductance levels.